In cable television networks (CATV), the CATV signal suffers from losses during transmission in cables and/or fibers. The attenuation characteristics of cables and/or fibers may increase as the frequency increases. On the other hand, attenuation of signals may increase as the length of cables and/or fibers increases. Different attenuation slopes would be generated for transmission of signals by transmission cables and/or fibers over different distances. Therefore, equalization circuits of different slopes are needed to equalize the attenuation of signals and balance in-band curves such that signals can be transmitted stably with high quality over a long distance.
In downlink amplifiers and nodes, interstage equalization circuits are usually arranged between an intermediate amplifier and a final-stage amplifier of the product. Conventional equalization circuits employ the following schemes. (1) An attenuator of a fixed slope can be used. A detachable equalization module includes an attenuator and peripheral circuits. Equalization modules of different slopes can be substituted in for different transmission cables and/or fibers over different distances. (2) An in-line attenuator of a fixed slope can be used. The peripheral circuits of the in-line attenuator are not integrated with the in-line attenuator, but arranged on a main board. The slope can be changed by detaching or attaching different in-line attenuators from or on the main board to achieve the purpose of compensation. The second scheme is more cost-effective compared to the first, but both schemes require equalization modules or in-line attenuators of different slopes. It is inconvenient to operate according to these schemes, and the demand for the equalization modules or in-line attenuators will also greatly increase as the number of amplifiers and nodes used increases, requiring a great amount on cost and maintenance. (3) A third scheme involves an electric control equalization circuit as shown in FIG. 1. The electric control equalization circuit comprising a digital step attenuator 1a and peripheral circuits is employed. The signal slope of the digital step attenuator 1a of the electric control equalization circuit can be changed remotely or locally with a control module, so that the slope of signal attenuation can be compensated. The control module can be conventional means such as a microcontroller, or a DSP. The third scheme offers substantial enhancements over the first and second schemes in maintenance costs and operation.
However, equalization circuits of existing cable television networks still have the following technical problems: Since the slope of an electric control equalization circuit may swing between 0 dB and 22 dB, when the slope of a digital step attenuator 1a changes, the change of impedance and the intrinsic frequency response characteristics of the circuit would cause the flatness of the signal waveforms of the digital step attenuator 1a for different slopes to vary substantially, affecting the characteristics of the transmission signal.